1. Field of the Invention
This invention relates to an apparatus for inspecting a foreign substance such as minute dust or a flaw adhering to an object to be inspected by the scanning of a light beam, and in particular to a system and method for inspecting a foreign substance adhering to a photomask or a reticle used for the manufacture of semiconductor integrated circuits.
2. Description of the Prior Art
A foreign substance (such as minute dust or a flaw) adhering to a photomask or a reticle (hereinafter typically referred to as a mask) for the manufacture of IC or LSI, like a pattern such as chromium intercepting the light for exposure, is transferred to a photoresist layer on a semiconductor wafer (hereinafter referred to as the wafer). This means that unsatisfactory operation of the manufactured semiconductor integrated circuit or a significant reduction in yield may result. Thus, as disclosed, for example, in U.S. Pat. No. 4,468,120, there is proposed an apparatus in which a light beam such as laser light is scanned on a mask and the light information from a foreign substance, particularly, scattered light of weak directionality, is photoelectrically detected by photoelectric detectors disposed in a plurality of directions and on the basis of the photoelectric signals thereof, the presence of the adherence of a foreign substance, the state of adherence thereof, the size of the foreign substance, etc. are automatically inspected. The accuracy of foreign substance detection in apparatus of this type is determined by the intensity of the laser light, the spot size of the laser light on the mask or the sensitivity of the photoelectric detectors (for example, photomultipliers). If this foreign substance detection accuracy is not constant during each inspection, the result of the inspection will vary greatly for each mask in spite of the fact that foreign substances of the same size and shape adhere to different masks.